1. Field of the Invention
The present invention generally relates to an optical cable comprising optical fibers identifiable by coloring, to an optical fiber identifiable by coloring, to a method for identifying an optical fiber and to the manufacturing of optical cables comprising identifiable optical fibers.
2. Description of the Related Art
An optical fiber typically comprises a core, generally made of glass, and a surrounding cladding; the core and the cladding form the optical waveguide structure, and hereinafter they will also be collectively referred to as the “transmissive element” of the optical fiber.
One or more coating layers are typically applied over the cladding. For example, two superposed coating layers are often provided. The layer which is in direct contact with the transmissive element is usually called inner coating (or primary coating), whereas the overlaying layer is called outer coating (or secondary coating).
Typically, like in the telecommunications field, optical fibers are used in optical cables, i.e. they are “cabled”. Optical cables are used, for example, for connecting customer premises to an optical communications network.
Optical cables generally comprise a plurality of optical fibers, contained in, e.g., buffer tubes, preferably loosely confining the optical fibers in a number for instance variable from 6 to 48 or even more, depending on the specific application. The optical fibers may optionally be gathered in groups of, for example, more than 12.
For sake of simplicity and unless otherwise stated, in the following the term “optical cable” shall collectively refer to optical cables themselves, but also to buffer tubes.
After cabling, the individual fibers should be still readily distinguishable from each other so they can be accurately identified during, for example, installation (connecting ad splicing) and repair.
Color coding can be used to distinguish and identify individual fibers in a multi-fiber optical cable. For example, a colorant can be added to the coating materials, usually to the outer coating material; alternatively, an additional layer containing a colorant can be provided at least partially overlaying the fiber outer coating.
According to TIA-598-C (Telecommunication Industry Association) color specifications, a dozen of different colors can be used to discriminate the fibers, thus when the number of fibers inside a cable is large, particularly greater than 12, the limited number of colors available may result insufficient, and additional implements are required for identifying the fibers.
Current solutions are known which are based on i) adding indicia, such as rings or stripes, over or in the colored layer; or ii) using colored yarns to segregate groups of fibers within the optical cable.
For example, U.S. Pat. No. 6,904,212 discloses a method for coloring an optical fiber. The method comprises applying a base color coating of a first curable material having a base color to an optical fiber core section. At least one stripe of a second curable material is applied to the base color coating before these coatings are fully cured. The stripe has a stripe color different from the base color. The stripe is preferably formed in the base color coating such that a diameter of the colored optical fiber remains substantially uniform.
U.S. Pat. No. 6,404,972 relates to a color-coded optical fiber with a fiber core and the cladding made e.g. of a quartz glass, having a coating, which comprises one or several plastic casings to protect the fiber. Immediately above this coating is a color coding in the form of open colored rings spaced in the perimeter direction. In order to let such an open colored ring appear optically as a full ring for the better identification of each fiber, and at the same time to positively prevent any wear of the color coding during further processing, a transparent or translucent layer is provided which covers the color coded fiber along its entire length. Further color variations and thereby further differentiating possibilities are provided if the further layer is colored or dyed. In this way perhaps black or blue single or double rings can be covered by a yellow, red or green-dyed further layer of resin. The colored and essentially open single or double rings can be easily identified as full rings since the colored further layer is at least translucent. The selection of contrasting color combinations facilitates the identifiability of the coded fiber.
U.S. Pat. No. 5,796,905 discloses an optical fiber with a color marking. An optical fiber has a fiber core and cladding made for example of quartz glass, having a coating, which is composed of one or more plastic coatings to protect the fiber. A color marking in the form of spaced colored rings is placed directly on this coating. A further transparent or translucent layer, which covers the entire length of the color-identified fiber, is provided. Further color variations, and thereby further distinguishing possibilities, are provided if the further layer is colored or dyed. The colored single or double rings can be easily identified, since the further colored layer is at least translucent. The selection of contrasting color combinations makes the marked fiber more identifiable. Another embodiment provides a fiber made of glass or plastic covered by a coating that in turn contains the color marking. This color marking comprises lengthwise running dashes or lines of any color, where this marking is either applied directly to the coating or is applied thereto if a uniform lengthwise extending color marking, which itself is colored, is applied to the coating. Like the colored rings, the dash or line markings can contrast against the surface of the plastic used for the coating, but the dashes or lines can also be integrated into the plastic material. In all instances, the further layer is provided to protect the color marking, and for the purpose of precisely distinguishing between the multitude of optical fibers in cables by means of any marking configurations. In another embodiment the optical fiber is surrounded by the coating, specifically a “coating” in the case of glass fibers, to which the color marking is applied directly, or over an intermediate layer which is colored throughout. In the illustrated embodiment, it comprises spaced dashes or lines. These broken dashes or lines, possibly at different distances from each other, are covered by the further layer, which covers the entire optical fiber. Another differentiation of fibers marked in this manner is provided, if the further layer is made of a translucent resin, the color of which can be distinguished from the colors or color combinations located underneath.
U.S. Pat. No. 5,377,292 relates to an optical fiber with one or more protective coverings of polymeric materials and a color coating of UV-hardened varnish on an outer surface of a last protective covering, wherein a different color identification marking material is interposed between parts of the color coating before hardening.